The idea of applying a nitride semiconductor to a high-voltage, high-power semiconductor device by taking advantage of features such as a high saturated electron velocity and a wide band gap is under consideration. For example, GaN that is a nitride semiconductor has a band gap of 3.4 eV larger than the band gap (1.1 eV) of Si and the band gap (1.4 eV) of GaAs and has a high breakdown electric field strength. For this reason, GaN is a very promising material for a power semiconductor device for high-voltage operation and high power.
Devices using a nitride semiconductor include field-effect transistors. There have been numerous reports on field-effect transistors, particularly high electron mobility transistors (HEMTs). For example, among GaN-based HEMTs (GaN HEMTs), an AlGaN/GaN HEMT using GaN in an electron transit layer and AlGaN in an electron supply layer is attracting attention. In an AlGaN/GaN HEMT, strain occurs in AlGaN due to the difference in lattice constant between GaN and AlGaN. Piezoelectric polarization resulting from the strain and spontaneous polarization of AlGaN lead to formation of a high concentration of two-dimensional electron gas (2DEG). Accordingly, an AlGaN/GaN HEMT is expected to serve as a high-efficiency switching device or a high-voltage power device for an electric vehicle or the like.    Patent Document 1: Japanese Laid-open Patent Publication No. 2009-076845    Patent Document 2: Japanese Laid-open Patent Publication No. 2007-019309    Non-Patent Document 1: Applied Physics Letters Biaxial strain-modified valence and conduction band offsets of zinc-blende GaN, GaP, GaAs, InN, InP, and InAs, and optical bowing of strained epitaxial InGaN alloys P. R. C. Kent, Gus L. W. Hart, and Alex Zunger National Renewable Energy Laboratory, Golden, Colo. 80401 Received 3 Jul. 2002; accepted 2 Oct. 2002
There is demand for a technique for locally controlling the amount of 2DEG generated in nitride semiconductor devices. For example, in the case of a HEMT, so-called normally-off operation in which no electric current flows when the voltage is off is desired from a so-called fail-safe standpoint. Realization of normally-off operation requires a contrivance to reduce the amount of 2DEG generated below a gate electrode when the voltage is off.
As one method for realizing a GaN HEMT which operates in a normally-off manner, there is proposed the process of forming a p-type GaN layer on an electron supply layer and counteracting 2DEG at a part below the p-type GaN layer to realize normally-off operation. In the process, p-type GaN is grown across a surface of, e.g., AlGaN serving as an electron supply layer, the p-type GaN is dry-etched such that a part where a gate electrode is to be formed is left and forms a p-type GaN layer, and a gate electrode is formed on the p-type GaN layer.
The activation rate of p-type GaN is low. For this reason, to cause a p-type GaN layer to generate a carrier enough to counteract a portion of 2DEG corresponding in position to the p-type GaN layer, the p-type GaN layer needs to be correspondingly thick. The formation of the thick p-type GaN layer leads to difficulty in etching. The formation also leads to difficulty in gate control, which causes deterioration of device performance.